Abstract
Estimating equivalent permeability at grid block scale of numerical models is a critical issue for large-scale fractured porous rocks. However, it is difficult to constrain the permeability distributions for equivalent fracture models as these are strongly influenced by complex fracture properties. This study quantitatively investigated equivalent permeability distributions for fractured porous rocks, considering the impact of the correlated fracture aperture and length model. Two-dimensional discrete fracture models are generated with varied correlation exponent ranges from 0.5 to 1, which indicates different geomechanical properties of fractured porous rock. The equivalent fracture models are built by the multiple boundary upscaling method. Results indicate that the spatial distribution of equivalent permeability varied with the correlation exponent. When the minimum fracture length and the number of fractures increase, the process that the diagonal equivalent permeability tensor components change from a power law like to a lognormal like and to a normal-like distribution slows down as the correlation exponent increases. The average dimensionless equivalent permeability for the equivalent fracture models is well described by an exponential relationship with the correlation exponent. A power law model is built between the equivalent permeability of equivalent fracture models and fracture density of discrete fracture models for the correlated aperture-length models. The results demonstrate that both the fracture density and length-aperture model influence the equivalent permeability of equivalent fracture models interactively.
Highlights
Fractures are among the most common structures in the brittle rocks of the Earth’s crust and span a wide range of length scales from millimeter to kilometers
The methodology in the paper could be applied for efficient estimation of equivalent permeability distributions for fractured porous rocks correlating fracture length and aperture, which can be incorporated into stochastic equivalent fracture models
Previous studies mainly concentrate on the comparison between constant and varied aperture models on equivalent permeability of one grid block (e.g., [35]); this study investigates the influence of correlated fracture aperture and length model on equivalent permeability distributions, which helps link discrete fractures at the small scales to equivalent permeability of reservoir models at the field scales
Summary
Fractures are among the most common structures in the brittle rocks of the Earth’s crust and span a wide range of length scales from millimeter to kilometers. Permeability of fractured rocks can be estimated by laboratory experiment [10], borehole flowmeter [11], field pumping test [12], discrete fracture characterization data from core samples [13], borehole well logs [14], and outcrops [15]. Such measurements can be roughly classified into continuum approaches and discrete fracture approaches [16]. An understanding of the influence of discrete fractures on the equivalent permeability of grid blocks opens the Geofluids possibility of creating efficient and accurate equivalent fracture models based on the multiple-scale fracture characterization data
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